1. Field of the Invention
The present invention relates, in general, to pressure sensors capable of operating at high or elevated temperatures and more particularly to shape memory alloys (SMA's) that can be used at high or elevated temperatures as a pressure sensor.
2. Technical Background
In recent years there has been a need for high or elevated temperature pressure sensors for various applications including for use in harsh environments. In these harsh environments such as for use in engine cylinders and turbine engines, the pressure sensors are exposed to a corrosive, oxidizing environments which put high mechanical and thermal stresses on the sensors. Various approaches have been taken in order to protect the pressure sensors from these environmental conditions and to allow the sensor to remain operational over extended periods of time. These approaches include sealing the pressure sensor to shield it from the environment such as follows:
U.S. Pat. Nos. 6,363,792 and 6,530,282 to Kurtz et al. disclose a hermetically sealed high temperature pressure transducer assembly (sensor) formed on silicon including: a sensor wafer that includes a plurality of sensor structures and contact areas selectively interconnected and formed on a surface. The surface being hermetically sealed and isolated from the harsh environment from which the pressure is being measured.
U.S. Pat. No. 6,523,415 to Kurtz et al. discloses a pressure transducer formed on silicon including: a first surface adapted for receiving a pressure applied thereto, an oppositely disposed second surface, and a flexing portion adapted to deflect when pressure is applied to the first surface; at least a first sensor formed on the second surface and adjacent to a center of the flexing portion, and adapted to measure the pressure applied to the first surface, at least a second gage sensor formed on the second surface and adjacent to a periphery of the flexing portion, and adapted to measure the pressure applied to the first surface; a glass substrate secured to the second surface of the silicon wafer.
While sealing the sensor from the environment has helped create a more durable sensor, at high temperatures these sensing devices also suffer from the drawback of having too low of a gage factor resulting in larger sensors or sensors with signals that are difficult to measure. Gage factor is a measure of the sensitivity of the sensor. With too low of a gage factor, the sensitivity of the sensing element is reduced creating difficulty in reading the sensing element, or the diaphragm size has to be increased to make up for the reduced sensitivity. These devices are typically made by diffusing the sensing elements into a silicon diaphragm. With these types of devices the gage factor significantly decreases with increasing temperature.
This drawback has been avoided in part by the use of other materials. U.S. Pat. No. 6,327,911 to Kurtz discloses a high temperature pressure transducer comprising a diaphragm fabricated from beta-silicon carbide and at least one sensing element fabricated from beta-silicon carbide associated with the diaphram. Kurtz claims that this type of transducer exhibits gage factors of above 30 at room temperature and between 10-15 at 550° C. While the development by Kurtz improves the potential sensitivity, it would be desirable to have a sensor with even better sensitivity and that does not change significantly with temperature increases so that not only can the sensing element be read more easily, but also so smaller sensors can be formed.
In looking at other forms of materials, there is a material known as a shape memory alloy (SMA). Shape memory alloys (SMA's) are a class of materials that have the ability to form two different crystalline phases, usually referred to as the martensite and austenite phases, in response to temperature and strain. SMA's are produced by combining at least two component elements into a desired shape which is then annealed. Immediately upon being annealed, the SMA material is in the austenite phase, having a specific shape (referred to hereinafter as the parent shape), and characterized by a low ductility, high Young's Modulus and high yield stress. Upon cooling, the SMA material changes into the martensite phase characterized by a high ductility, low Young's Modulus and low yield stress. In the martensite phase, the SMA material is easily deformed and can take on a different shape from its austenite, or parent, shape by the application of an external strain thereto. The SMA material will retain this different shape until it is heated to its austenite phase transformation temperature. When such heating occurs, the SMA material undergoes a phase transformation to its austenite phase and is transformed back to its parent shape. During this phase transformation the SMA material produces a very high kinetic energy output per unit volume. Because of this, SMA's can generate a relatively large force over a longer displacement as compared to other materials of the same size. Additionally, because of the electrical resistance characteristics of SMA material, joule heating can be used to raise the SMA material to its austenitic phase transformation temperature. Furthermore, the electrical resistance characteristics of SMA material results in a strain-dependent electrical resistance effect at the phase transformation temperature.
The two significant physical properties of SMA material, i.e., high recoverable strain and high actuation energy densities, have led to the development of SMA materials and devices for various applications. Bulk or thick film SMA materials are produced using traditional metal forming processes and are incorporated into many different devices ranging from orthodontia appliances to visored helmets. In these applications the bulk or thick film SMA materials take the form of wires, springs, thread fasteners, ring clamps, etc. Thin film SMA materials are produced by depositing an alloy on a substrate and have gained acceptance in micro fluidics and temperature related applications, particularly as actuators. Typically, applications utilizing bulk or thick film SMA materials exploit the one-way shape-memory property of SMA material. In these applications the bulk or thick film SMA material is strained (deformed) in the low temperature martensite phase and recovers to its parent shape upon being heated to the temperature at which the SMA material is transformed to its high temperature austenite phase. The strain-dependent electrical resistance effect of SMA material, however, has not been utilized for strain measuring devices or sensors because of is the thermodynamic inefficiency of bulk or thick film SMA material. The hysteresis characteristics of bulk or thick film SMA material, which determine the phase transformation cycle period, are too slow (on the order of seconds) to be effective as a sensor. The slowness of the hysteresis characteristics of bulk or thick film SMA material is caused by the high thermal mass of this material. In contrast, due to the low thermal mass of thin film SMA material, the hysteresis characteristics of this material are quite fast (on the order of cycles/second) which makes thin film SMA material particularly suitable for certain applications, such as sensors, where the change in electrical resistance at a phase transformation of this material can be correlated to a change in strain being applied to the material. A problem, however, arises with thin film SMA material due to the difficulty in producing reliable thin film SMA materials that can repeatedly and consistently provide accurate strain measurements. Recent advances in manufacturing techniques, however, have resulted in the production of thin film SMA materials that exhibit a consistent quality suitable for use in strain measuring devices. Because of these manufacturing advances, it has become desirable to develop a sensor and method for measuring strain utilizing thin film SMA materials.
It is therefore the object of the present invention to provide a high temperature pressure sensor without the drawbacks of the prior art. It is further the object of the present invention to provide a high temperature pressure sensor with an improved gage factor. It is still further the object of the present invention to provide a pressure sensor with a smaller sized diaphragm, which is also capable of reading higher pressures. Finally, it is even still further the object of the present invention to provide a high temperature sensor made from a SMA material.